President Trump headed home on Saturday to confront a growing political and legal threat, as his top aides tried to contain the fallout from reports that his son-in-law, Jared Kushner, is a focus of investigations into possible collusion between Russia and the president’s campaign and transition teams.

As Mr. Trump ended a nine-day overseas trip that aides considered the most successful stretch of his presidency, he was returning to a crisis that had only grown in his absence. The White House cancelled a presidential trip to Iowa in the coming days and was putting together a damage-control plan to expand the president’s legal team, reorganise his communications staff and wall off a scandal that has jeopardised his agenda and now threatens to engulf his family.

Mr. Trump’s private legal team, led by his New York lawyer, Marc Kasowitz, was preparing to meet in Washington to face fresh questions about contacts between Mr. Kushner and representatives of President Vladimir V. Putin of Russia. The president may meet with Mr. Kasowitz as early as Sunday, and aides have recruited a series of prominent Washington lawyers with experience in political investigations for Mr. Trump to interview in hopes that they might join the legal team.

Mr. Kushner, who organised the president’s Middle East stops at the start of the foreign trip, chose to return to Washington with several days yet to go and has been unusually subdued since then. But he has no plans to step down from his role as senior adviser or to reduce his duties, according to people close to him.

Still, there are signs that he is tiring of the nonstop combat and the damage to his reputation. He has told friends that he and his wife have made no long-term commitment to remain by Mr. Trump’s side, saying they would review every six months whether to return to private life in New York.

Mr. Kushner’s troubles are only one facet of the crisis. Reince Priebus, the White House chief of staff, and Stephen K. Bannon, the president’s chief strategist, also dropped off Mr. Trump’s trip early, in part to return to deal with the political furor over the Russia investigations and the president’s decision to fire James B. Comey as F.B.I. director.

The White House was trying to figure out how to respond to reports that Mr. Kushner had spoken in December with Russia’s ambassador, Sergey I. Kislyak, about establishing a secret channel between his father-in-law’s transition team and Moscow to discuss the war in Syria and other issues. The Washington Post first reported on the suggestion on Friday, and three people informed about it confirmed it to The New York Times.

The discussion took place at Trump Tower at a meeting that also included Michael T. Flynn, who served briefly as Mr. Trump’s national security adviser until being forced out when it was revealed that he had misled Vice President Mike Pence and others about a separate telephone conversation he had with Mr. Kislyak. It was unclear who first proposed the secret communications channel, but the idea was for Mr. Flynn to speak directly with a Russian military official. The channel was never set up.

As reports emerged about investigators’ focus on Mr. Kushner, he and his wife, Ivanka Trump, discussed the possibility of having Donald F. McGahn II, the White House counsel, issue a statement denying that Mr. McGahn had been contacted by federal officials about Mr. Kushner. Mr. McGahn, who has been increasingly uneasy in his role since Mr. Trump ignored his advice to delay Mr. Comey’s dismissal, said he was not the person to write such a statement, suggesting that doing so would create a precedent requiring a response to each new report. Mr. Kushner’s private lawyer issued a statement instead.

Ms. Trump and Mr. Kushner have complained privately about what he views as an unfair level of scrutiny of his actions. He has dismissed the attention on him as a reflection of his father-in-law’s unconventional approach to diplomacy and inexperience in government, rather than anything nefarious he has done. People close to Mr. Kushner were adamant on Saturday that he was preparing for a long fight and not an exit from the White House.

The reports about Mr. Kushner dominated an end-of-trip briefing for reporters in Taormina, Italy, where Lt. Gen. H. R. McMaster, the president’s national security adviser, and Gary D. Cohn, his national economics adviser, declined to comment specifically on Mr. Kushner but sought to play down the significance of the disclosures.

“We have back-channel communications with any number of countries,” General McMaster said. “So, generally speaking, about back-channel communications, what that allows you to do is to communicate in a discreet manner. It doesn’t predispose you to any kind of content in that conversation.”

He did not say whether he was comfortable with the idea of a private citizen, as Mr. Kushner was at the time, opening such a back channel.

Behind the scenes, Mr. Trump’s advisers were working to create a crisis-control communications operation within the White House to separate the Russia investigations and related scandals from the administration’s day-to-day themes and the work of governing, according to several people familiar with their plans and who spoke on the condition of anonymity because they were not authorized to divulge the details of a still-evolving strategy.

The goal, these people said, was to give Mr. Trump more outlets for communicating his message in an unvarnished way, while curbing opportunities for aides to be confronted publicly with damaging developments or unflattering story lines.

White House aides were trying to assemble a powerhouse outside legal team that they hoped would include seasoned Washington lawyers of the stature of Paul D. Clement, Theodore Olson or Brendan Sullivan, and they planned to introduce some of them to Mr. Trump as soon as this weekend. More lawyers could also be hired onto the White House staff to help Mr. McGahn.

The approach is modelled on the war room used by President Bill Clinton during various inquiries, including one that led to his impeachment for lying under oath about his affair with Monica Lewinsky. Mr. Clinton retained a private legal team and established a separate office to handle questions about investigations so that the White House could preserve the image of governing and keep its primary focus on the president’s broader message.

Aides are talking about bringing Corey Lewandowski, Mr. Trump’s former campaign manager, and David Bossie, his former deputy campaign manager, onto the White House staff to manage the war room.

Under the evolving scenario, Sean Spicer, the White House press secretary, would take a diminished public role, with daily on-camera briefings replaced by more limited interactions with journalists, while Mr. Trump would seize more opportunities to communicate directly with his core supporters through campaign rallies, social media appearances such as Facebook Live videos, and interviews with friendly news media outlets.

The president, who has more than 30 million followers on Twitter, has been told by his lawyers to limit his posts. Each one, they argue privately, could be used as evidence in a legal case against him, and the president went through his entire overseas trip without posting a single incendiary message.

Among those most adamant about limiting Mr. Trump’s access to the news media was Mr. Kushner, who has been critical internally of the White House press operation and has sought to marginalizeMr. Spicer, whom he views as too undisciplined to control the president’s message. Mr. Kushner has also favoured creating a rapid-response team to counter reports like the ones that emerged on Friday.

In a move that many in the West Wing viewed as emblematic of his attempt to wrest control of communications from Mr. Spicer and Mr. Priebus, Mr. Kushner displaced an operations official from the office across the hall from his own and installed his personal spokesman, Josh Raffel, in his place, according to two people familiar with the matter.

Republican strategists said it was vital for Mr. Trump to focus on advancing a legislative agenda to show voters that the administration could deliver policy changes and allay lawmakers’ simmering fears that the president’s troubles could damage their re-election chances.

“What they need to do is crank up the legislative side of things and say, O.K., that’s going on, and Trump’s going to be Trump, but meanwhile, they’re actually working to get some kind of health care plan through the Senate, some kind of tax reform, and do what they promised they would,” said Rich Galen, a top adviser to Newt Gingrich when he was the House speaker during Mr. Clinton’s tenure.

Yet Mr. Trump’s push to revamp health care has faltered in Congress, and the White House has yet to present a detailed plan for his promised tax cuts.

Joel Johnson, who was a top adviser to Mr. Clinton, said the traditional options available to a besieged president returning from overseas would be delivering a major speech, shaking up his staff and getting out on the road. But he said it was unclear whether Mr. Trump could do any of those things effectively enough to recapture control of his own narrative.

“You’re always looking for a reset button and how do we change the conversation,” Mr. Johnson said. “I wouldn’t want to be running the program down there right now. I don’t know where they go.”